1. Field of the Invention
This invention relates to telephone subscriber loop testing systems and, in particular, to apparatus for determining whether a fault in the subscriber loop is located on one side of the apparatus or the other.
2. Description of the Prior Art
It is useful to know whether a fault in a telephone subscriber loop is located within a customer's premises or outside the premises so that an appropriate repair/person may be assigned for correcting the fault. A typical method of locating such a fault is disclosed in U.S. Pat. No. 3,636,280 issued Jan. 18, 1972 to Mr. L. E. Wetzel. At the entrance of the subscriber loop to the customer's premises, the subscriber loop is mechanically opened by a switch in response to a signal from a testing source. Thereafter, the opened loop is connected, for the duration of the test, to an impedance termination network.
Another such device is disclosed in U.S. patent application, Ser. No. 273,975, filed June 15, 1981, now U.S. Pat. No. 4,373,121 by Messrs. E. F. Sartori and F. J. Uhrhane, and assigned to the same assignee herein. The Sartori et al application discloses the use of a four-layer diode switch connected permanently in series with each loop conductor and substantially identical impedance termination networks connected permanently between each loop conductor and ground. A problem with such an arrangement arises when the resistance values in the termination networks are changed by lightning strikes because of the ground connections. Furthermore, because the four-layer diode switches cannot be produced, currently, for consistent performance, relaxation oscillation may occur during telephone conversations or signaling and supervision procedures.